Generally, a steel manufacturing plant generates gases which are harmful to the human body, and among these gases, CO gas is a dangerous one which can give a fatal inflict to the human body even at a concentration of less than 2000 ppm.
Conventionally there have been developed various CO gas detecting sensors for detecting the existence and absence of CO gas, but the SnO.sub.2 series CO gas detecting sensor which is currently used operates in the following manner. That is, an oxygen ion (O.sup.-) which has been adhered on the surface of the gas is desorbed by being reacted with a reactive gas, and therefore, the electron which has been captured by the oxygen ion is converted to a free electron, with the result that the conductivity of the gas is increased. This conductivity is measured, and the existence of CO gas is determined. However, this CO gas detecting sensor has a disadvantage that it is reacted with even hydrogen gas or propane gas (Japanese Chemical Journal 1987, No. 3, p447-488).
Meanwhile, recently, there has been developed another CO gas detecting sensor in which a p type semiconductor (CuO) and an n type semiconductor (ZnO) are physically contacted. According to Japanese Patent Laid-open No. Sho-62-90529, the CO gas detecting sensor has the advantage that it has the selectivity of reacting only with CO gas. However, it consists of two different semiconductors contacted together, and the two semiconductors are separated after the measurement. Therefore, when CO gas is to be measured again, a perfect contact realization of the two semiconductors are very low, with the result that the sensitivity for the measurement of CO gas is apt to be varied.